Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These systems operate in unlicensed spectrum. The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) specifications refer to D2D communications as sidelink communications.
D2D communication as an underlay to cellular networks allows devices to operate in a controlled interference environment and may also take advantage of the proximity of communicating devices, sometimes referred to as proximity services (ProSe). Typically, D2D communication shares the same spectrum as the cellular system. For example, a portion of the cellular uplink resources may be reserved for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is not a likely scenario, however, because spectrum is a scarce resource. Dynamic sharing between the device-to-device services and cellular services is a more likely scenario because it is more flexible and provides higher spectrum efficiency.
D2D transmissions may include various transmissions modes. For example, D2D transmission modes include unicast (i.e., a specific user equipment (UE) is the receiver), multicast or groupcast (i.e., a group of UEs are the receivers), and broadcast (i.e., all UE are the receivers).
Where there is no cellular network D2D communication (e.g., devices are out-of-coverage or off-network), data may be sent from one device to another device without prior arrangement. This may reduce transmission overhead and increase the communication capacity, which is crucial in emergency situations. A source device transmits data to one (unicast) or more (multicast/groupcast/broadcast) other devices, without first ensuring that the recipients are available and ready to receive the data. Such communication may be used for one-to-one or one-to-many communication, but it is particularly effective for multicast and broadcast transmissions. Thus, it is well-suited for broadcast and group communication. The communication may be realized, for example, via PHY unicast/multicast/groupcast/broadcast transmissions. Even using PHY broadcast transmissions, higher layers may still treat the transmissions as unicast/groupcast/multicast transmissions. For example, in the MAC layer, multicast or even unicast addresses may be used. Alternatively, if using broadcast on both PHY and MAC, multicast or unicast IP addresses may be used at the IP layer.
One way to provide D2D communication is to send a scheduling assignment (SA) followed by a data transmission. SAs are control messages used for direct scheduling of D2D communication. SAs are transmitted by a UE that intends to transmit D2D data, and they are received by UEs that are potentially interested in such data. A UE transmits SAs on dedicated resources characterized by time and frequency. These are typically a sparse resource. SAs provide information that the receiver can use to decode the D2D data transmission associated with the SA (e.g., the resources for data transmission, the modulation/coding parameters, timing information, identities for the transmitter and/or receiver, etc.). Typically, but not necessarily, SAs are transmitted prior to the actual data transmission, so that a receiver is able to selectively receive data based on the content of the SAs. The data transmissions scheduled by a SA may be referred to as a “transmission pattern.”
For data transmission between UEs that are both out-of-coverage or operating off-network (e.g., a situation where the UEs control the transmission with little or no help, such as scheduling, from the network), UEs may be preconfigured with certain parameters such as resource pool information (e.g., time and frequency configuration) used for data transmission. When UE-A has data to transmit to UE-B, UE-A typically sends a sync signal, which UE-B may use as a time reference. Then UE-A transmits a scheduling assignment followed by the actual data.
Some scenarios assume that the frequency resource for data transmission is the same frequency resource as for transmitting scheduling assignments. The time resource for data is provided by the time-pattern information element in the SA itself.
A UE may support discontinuous reception (DRX) by monitoring the identities carried in the SA. For example, for multicast D2D communication, the identity in the SA identifies the multicast group. Thus, a UE which is interested in receiving data for one or several multicast groups only needs to check the SAs for the corresponding identities. When the UE receives an SA with an identity corresponding to a multicast group of interest to the UE, the UE may decode the data transmission associated with the SA.
When the transmitting and receiving UEs are within coverage of a cellular network, the communicating UEs cannot rely on the aforementioned pre-configuration because the D2D transmissions need to coexist with potential legacy uplink transmissions. Therefore, the transmitting D2D UE (e.g., UE-A described above) requests transmission resources from a base station, such as an eNB. As part of requesting the transmission resources, the UE notifies the eNB that the UE has data to send by transmitting a buffer status report (BSR).
D2D communication defines resource pools to support both communication and discovery. The pools for communication may also be referred to as transmission pools or data transmission pools. A UE can be configured with multiple transmission pools. More specifically, at any given time a UE has up to 4 mode-2 SA and data transmission pools that may be available for selection at L1.
The layers above L1 need to specify how to use the L1 resource pools. As one example, mission critical prioritization of public safety communication can benefit from ProSe Group Priority. Current strategies for prioritizing resource pools comprise static mappings of priorities to pools. A risk with these strategies is that certain pools may be underutilized, or not used at all. Another risk is that certain pools may be over utilized. For example, certain pools may carry too much traffic, resulting in congestion, even though resources are available in other pools.